The present invention relates to a method for operating a tape storage system, a tape head assembly and a tape storage system.
A conventional tape storage medium typically comprises servo information prerecorded in dedicated servo bands that extend next to data bands for storing data, which servo bands and data bands extend in a longitudinal direction along the tape medium. For example, according to the LTO (Linear Tape Open) Ultrium specification four data bands are straddled by five servo bands. For reading and/or writing data from/to a data band a tape head assembly is positioned to span the entire width of the data band and at least parts of the two servo bands adjacent to the data band. The tape head assembly comprises a set of data readers and writers, e.g., sixteen or thirty-two, arranged next to each other along a lateral extension of the tape head assembly orthogonal to the longitudinal extension of the tape storage medium when being inserted in a tape drive containing the tape head assembly. The tape head assembly further comprises at least two servo readers at the edges of the set of data readers and writers, so that the data readers and data writers are located between the servo readers. The servo readers are provided for reading servo information from two adjacent servo bands. The servo information in the servo bands typically allows for determining several essential servo parameters, e.g., a lateral position of the tape storage medium with respect to the tape head assembly, a tape velocity, and a longitudinal tape position.
For writing data to the data band the tape head assembly is set to a desired lateral position on the tape storage medium and data is written simultaneously by the data writers of the set while the tape storage medium is moved across the tape head assembly in a first direction, e.g., in a forward motion direction. When the tape head assembly has reached the end of the tape storage medium, the tape head assembly is set to a different lateral position, i.e., laterally offset to the previous position, the tape motion direction is reversed and data is continued to be written simultaneously by the data writers of the set while the tape storage medium is moved across the tape head assembly in the reverse motion direction. Hence, the data band is recorded in a serpentine fashion.
A data track is referred to as a longitudinal track within a data band written by a single data writer. In serpentine recording, a set of data tracks written at the same time is referred to as a wrap. In serpentine recording, a set of data tracks written by the same data writer within the data band is also referred to as a data sub-band. The number of data tracks contained in a data sub-band, which also represents the number of wraps in the data band, depends on a width of the data band, a width of the data writers and a resolution of lateral position information that is enabled by the servo patterns in the servo band. In order to enable writing in the serpentine fashion, the data writers are arranged offset from each other on the tape head assembly along its lateral extension. A distance between two adjacent data writers may be set to the number of wraps to be recorded in the data band times the width of a data track such that data tracks written in the same wrap are offset from each other for leaving a gap in between for writing data tracks in subsequent wraps into the gap.
The servo information prerecorded in the servo bands allows for identifying a lateral position of the tape storage medium with respect to the tape head assembly when being read by the dedicated servo readers. Based on the identified lateral position the tape head assembly may also be set to a new desired lateral position at the end of a wrap for writing a new wrap in a reverse motion direction of the tape storage medium. In addition, the prerecorded servo information allows for determining a lateral deviation of the tape storage medium from a desired lateral position during writing and/or reading data. For this purpose, a position error signal representing the lateral deviation may be translated into a control signal for an actuator for controlling a lateral position of the tape head assembly with respect to the tape storage medium.
In an alternate way of writing data to a tape storage medium, which is also referred to as adjacent track recording, the tape head assembly being a part of a tape head and containing a transducer array no longer spans the entire width of a data band as defined above but only spans a portion of the width of the data band. Again, the tape head assembly may contain a set of data readers and writers which are now arranged next to each other along a lateral extension of the tape head assembly orthogonal to the longitudinal extension of the tape storage medium when being inserted in a tape drive containing the tape head assembly. While in serpentine recording the data writers of the set are arranged laterally offset from each other the data writers are now preferably arranged close to each other without a significant gap in between such that data tracks written simultaneously by the data writers are arranged adjacent to each other in the data band. Given that no additional data tracks are to be recorded in between two adjacent data tracks there is no need for a significant gap in between. Hence, data written by adjacent data writers result in adjacent data tracks on the data band while in serpentine recording data tracks written by adjacent data writers in one pass result in data tracks gapped from each other. In the latter case, the gap is filled or respectively will be filled by data tracks written or to be written in different wraps.
In adjacent track recording the set of data tracks simultaneously written next to each other during the tape storage medium moving across the tape head assembly is also referred to as a data sub-band. The number of data sub-bands within a given width of the tape depends on the width of the tape head assembly. Once a data sub-band is written and the tape head assembly has reached the end of the tape storage medium, the tape head assembly is set to a new lateral position with respect to the tape storage medium for writing another data sub-band in a reverse tape motion direction. The offset of the new lateral position to the previous lateral position is at least the width of the tape head assembly to avoid overwriting of the previously written data sub-band.
An advantage offered by adjacent track recording is given by the reduced width of the tape head assembly compared to the width of the tape head assembly in the serpentine recording. By having reduced the total span of the tape head assembly and making a channel pitch, i.e., a pitch between adjacent data readers or writers, equal to the track pitch, environmental effects known as tape dimensional stability effects can be reduced. For example, the tape storage medium may laterally expand or contract dependent on variations in temperature and/or in humidity. Such expansion or contraction may more adversely impact the writing and reading of data the larger the tape head assembly span is.